Volumetric ultrasound imaging system using two-dimensional array transducer

ABSTRACT

Volumetric ultrasound images are created using a two-dimensional array transducer to create multiple beams that diverge in an elevational direction and scan in an azimuthal direction. In one embodiment, ultrasound echoes in three beams positioned adjacent each other in the elevational direction are projected onto respective planes. The volumetric image is created by combining the planes of projection for all three beams. The area scanned by the transducer is divided into three beams so that echoes located at the same distance from the transducer are at substantially the same depth beneath the transducer. In another embodiment, multiple beams scan in respective ranges of scanning depths, and the elevational divergence angle is reduced for deeper ranges of scanning depths. As a result, the elevational width of the volumetric image can be relatively constant. In another embodiment, multiple intersecting or parallel beams are used to create volumetric images.

This invention claims the benefit of Provisional U.S. patent ApplicationSer. No. 60/528,797, filed Dec. 11, 2003.

TECHNICAL FIELD

This invention relates to ultrasound imaging systems, and, moreparticularly, to a system and method for performing volumetric imagingusing a two-dimensional transducer that scans using multiple fan-shapedbeams.

BACKGROUND OF THE INVENTION

Various noninvasive diagnostic imaging modalities are capable ofproducing cross-sectional images of organs or vessels inside the body.An imaging modality that is well suited for such real-time noninvasiveimaging is ultrasound. Ultrasound diagnostic imaging systems are inwidespread use by cardiologists, obstetricians, radiologists and othersfor examinations of the heart, a developing fetus, internal abdominalorgans and other anatomical structures. These systems operate bytransmitting waves of ultrasound energy into the body, receivingultrasound echoes reflected from tissue interfaces upon which the wavesimpinge, and translating the received echoes into structuralrepresentations of portions of the body through which the ultrasoundwaves are directed.

In conventional ultrasound imaging, objects of interest, such asinternal tissues and blood, are scanned using planar ultrasound beams orslices, which are preferably as thin as possible to provide goodresolution of such objects accompanied by minimal clutter. A lineararray transducer is conventionally used to scan a thin slice by narrowlyfocusing the transmitted and received ultrasound in an elevationaldirection and steering the transmitted and received ultrasoundthroughout a range of angles in an azimuthal direction. A linear arraytransducer operating in this manner can provide a two-dimensional imagerepresenting a cross-section through either a plane that isperpendicular to a face of the transducer for B-mode imaging or parallelto the face of the transducer for C-mode imaging.

Although B-mode and C-mode images are two-dimensional images, it is alsopossible to generate three-dimensional ultrasound images by eitherphysically moving a linear array or by using a two-dimensional arraytransducer to steer the transmitted and received ultrasound about twoorthogonal axes. Although two-dimensional B-mode or C-mode images canconventionally be generated at a sufficient rate to allow essentiallyreal-time imaging (i.e., at least about 30 frames per second), it isgenerally not possible at the present time to generate three-dimensionalultrasound images at a rate that is sufficient to permit real-timeimaging. Three-dimensional real-time imaging poses two major challenges:first, acquiring echoes from a volume in a sufficiently short time tomaintain a real-time image frame rate, and, second, reducing volumetricdata obtained from these echoes to a suitable two-dimensional imageformat with sufficient speed to provide real-time display.

One technique that has been developed to create ultrasound imagesproviding information about anatomical structures in a three-dimensionalvolume is volumetric imaging, as disclosed in U.S. Pat. No. 5,305,756,which is incorporated herein by reference. Volumetric imaging cangenerally be accomplished at a sufficient speed to permit real timeimaging. With reference to FIG. 1, volumetric imaging is accomplishedusing a transducer 10 having linear array elements 12. The transmittedand received ultrasound is focused in the azimuthal direction AZ.However, lenses placed on the surface of the elements 12 or the surfacegeometry of the element 12 themselves cause the ultrasound to diverge inthe elevation direction EL to generate a series of fan-shaped beams,collectively shown as 14. The transducer 10 is scanned in a linear arrayformat whereby the ultrasound is sequentially transmitted and receivedfrom each array element 12 to form the sequence of fan-shaped beams 14.The beams 14 are orthogonal to the longitudinal surface of thetransducer 10 to insonify a volumetric region. In the center of theinsonified volumetric region is a plane of projection 18 that bisectseach of the fan-shaped beams 14. The plane of projection 18 is spatiallyrepresented by the ultrasound image produced by the transducer 10 and isa plane that typically is normal to the surface of the transducer 10 inthe azimuthal direction. The resulting ultrasound image providesinformation about the entire three-dimensional volumetric region becausethe transducer 10 acoustically integrates all echoes at each rangeacross the entire volumetric region. These echoes are then projected orcollapsed onto the plane of projection 18. Since the fan-shaped beams 14diverge radially in the elevation direction, each constant range locusis a radial line as indicated by a constant range locus 20. Each echoalong the constant range locus 20 is projected to a point 22 ofintersection of the locus 20 and the plane of projection 18. Since thisprojection occurs at every range and azimuthal location throughout thevolumetric region 16, the image of the plane of projection 18 presents atwo-dimensional projection of the entire volume. The resulting image issimilar to the two-dimensional projection of a volume obtained usingconventional x-ray imaging.

The volumetric image can be obtained as shown in FIG. 1 in essentiallyreal time because all of the echoes at each range across the entirevolumetric region isonified by each beam 14 are processed as a singlepoint on the plane of projection 18. As a result, relatively littleprocessing power is required, particularly compared to truethree-dimensional ultrasound imaging.

While the transducer 10 may be scanned in a linear array format as shownin FIG. 1 to form a sequence of fan-shaped beams, the transducer 10 mayalternatively used by transmitting and receiving properly phasedultrasound signals to and from the array elements 12. By operating thearray elements as a phased array, the transducer 10 can electronicallysteer and focus the ultrasound as shown in FIG. 2. The ultrasound istherefore transmitted and received in a fan-shaped beam 30 that divergesin both the elevational and azimuthal directions. The electronicsteering of the beam 30 enable the isonification of a pyramidal shapedvolumetric region adjacent the transducer 10. Ultrasound echoes fromwithin this volumetric region are projected onto a triangular shapedplane of projection 36 and used to display a volumetric image.

FIG. 3 illustrates another technique that is described in U.S. Pat. No.5,305,756 to produce of a fan-shaped beam in the elevational direction.As shown in FIG. 3, a transducer 40 has array elements 42 arranged intwo dimensions. As in the transducer 10 of FIGS. 1 and 2, the arrayelements 42 are aligned in the azimuthal direction. However, each arrayelement 42 is sub-diced in the elevational direction to formsub-elements 46 a,b,c. The sub-elements 46 a,b,c aligned in theelevational direction allows a series of fan-shaped beams 48 thatdiverge in the elevational direction to be electronically generatedrather than relying upon lenses or the geometry of the element surfaceto generate a fan-shaped beam. The sub-elements 46 a,b,c generate thefan-shaped beams 48 by controlling the time that signals are sent to orreceived from the sub-elements 46 a,b,c. For example, the sub-element 46b could be actuated first, followed in rapid succession by thesimultaneous actuation of the sub-elements 46 a and 46 c. However, it isimportant to note that the sub-elements 46 a,b,c are not used as aphased array in which properly phased ultrasound signals are transmittedfrom and received by the sub-elements 46 a,b,c. Thus, the beams 48 arenot steered in the elevational direction. As with the previouslydescribed embodiments, the ultrasound echoes in the volumetric regionisonified by the beams 48 are projected onto a plane 49 from which thevolumetric image is created.

Although the conventional volumetric imaging technique described aboverepresents a significant advance because it allows real time imaging ofa three-dimensional volumetric space, it is not without its limitations.For example, as illustrated in FIG. 4A, a transducer 50 shown whenviewed in the azimuthal direction scans using a diverging beam 52 asillustrated in FIGS. 1-3. When the transducer 50 is scanning to a rangeof distances 56 from the transducer 50, all of the points at that range56 from the transducer 50 will be projected onto a plane of projection60 as a set of points within a range of depths 62. Therefore, all of thepoints in that range of distances 56 from the transducer 50 will appearto be in the range of depths 62 on the projection 60 even though theactual depths of the points vary throughout a substantially larger range66. As a result, viewed in the elevational direction as shown in FIG.4B, a set of points in the range of depths 62 will be erroneouslyprojected to be within the range of depths 66. Conversely, an anatomicalstructure that spans a range of depths can appear to be at a singledepth because it is a constant distance from the transducer 50.

The problem exemplified by FIGS. 4A, 4B is exacerbated when theelevational divergence angle of the beam 52 is large. Under suchcircumstances, the volumetric image can fail to clearly show the trueconfiguration of anatomical structures.

Another problem with the conventional three-dimensional volumetricimaging technique shown in FIGS. 1-3 can be explained with reference toFIG. 5. FIG. 5 shows a transducer 80 viewed in the azimuthal directionthat is transmitting a beam 82 that diverges in the elevationaldirection, in the same manner as shown in FIGS. 1-3. The divergingnature of the beam 82 inherently means that the beam 82 will isonify anarea of interest beneath the transducer 80 that varies from a relativelysmall width near the transducer 80 to a relatively large width away fromthe transducer 80. For example, the beam 82 will isonify a width W₁ at adistance D₁ from the transducer 80, and will isonify a width W₂ at adistance D₂ from the transducer 80. Therefore, the resulting volumetricimage will be relatively narrow and show relatively little at the top ofthe image and will be relatively wide and show substantially more at thebottom of the image. The width of the image can be made equal bycropping the image, such as along lines 86, 88, but doing so wastesimage information that would otherwise be viewable.

Still another potential problem that may be encountered in using thethree-dimensional volumetric imaging technique shown in FIGS. 1-3 isthat certain regions of the image may not be shown in the image withsufficient clarity. For example, since the image does not resolveanatomical structures that lie along the same constant range locus fromthe transducer, a structure that occupies only a small portion of theconstant range locus may be obscured by other anatomical structures thatalso lie on the constant range locus.

There is therefore a need for a volumetric imaging system and methodthat clearly shows anatomical structures being imaged without geometricdistortion, and does so in a manner that can generate an image having asubstantially constant width throughout a range of depths.

SUMMARY OF THE INVENTION

A system and method of producing volumetric ultrasound images uses atwo-dimensional array transducer to scan a region of interest. Accordingto one aspect of the invention, the two-dimensional array transducerscans the region of interest in an azimuthal direction using a pluralityof beams that diverge in an elevational direction and are positionedadjacent each other in the elevational direction. Ultrasound reflectionsin each beam are projection onto a respective plane of projection, and avolumetric ultrasound image is then created by combining the projectionson the planes of projection for all of the beams into a common plane ofprojection.

According to another aspect of the invention, the two-dimensional arraytransducer scans the region of interest in an azimuthal direction usinga plurality of beams that have a common center axis. The beams divergein an elevational direction in respective divergence angles that aredifferent for each beam. The beams scan respective ranges of scanningdepths that are ordered inversely to an order of divergence angles ofthe beams. As a result, a beam scanning the shallowest range of scanningdepths has the largest divergence angle and a beam scanning the deepestrange of scanning depths has the smallest divergence angle. Theultrasound reflections in each beam are projected onto a common plane ofprojection, and the volumetric ultrasound image is created from theultrasound reflections projected onto the common plane of projection forall of the beams.

In still another aspect of the invention, the two-dimensional arraytransducer scans the region of interest in an azimuthal direction usinga pair of beams. A first beam diverges in a first direction and is usedto scan the region of interest in a second direction that isperpendicular to the first direction. Similarly, a second beam divergesin a third direction and is used to scan the region of interest in afourth direction that is perpendicular to the third direction.Ultrasound reflections in the first beam are projected onto a plane ofprojection that is perpendicular to the first direction, and ultrasoundreflections in the second beam are projected onto a plane of projectionthat is perpendicular to the third direction. A volumetric ultrasoundimage is then created from the first and second planes of projection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic isometric view illustrating one conventionaltechnique for generating volumetric images.

FIG. 2 is a schematic isometric view illustrating another conventionaltechnique for generating volumetric images.

FIG. 3 is a schematic isometric view illustrating still anotherconventional technique for generating volumetric images.

FIGS. 4A and 4B are schematic elevational and azimuthal cross-sectionviews, respectively, illustrating a limitation of the conventionalvolumetric imaging techniques shown in FIGS. 1-3.

FIG. 5 is a schematic elevational cross-section view illustratinganother limitation of the conventional volumetric imaging techniquesshown in FIGS. 1-3.

FIGS. 6A and 6B are schematic elevational and azimuthal cross-sectionviews, respectively, illustrating a technique for generating volumetricimages according to one embodiment of the invention.

FIG. 7 is a schematic elevational cross-section view illustrating atechnique for generating volumetric images according to anotherembodiment of the invention.

FIGS. 8A, 8B, 8C and 8D are schematic views illustrating techniques forgenerating volumetric images according to still another embodiment ofthe invention.

FIG. 9 is a block diagram of an ultrasound imaging system that can beused to perform volumetric imaging according to the embodiments shown inFIGS. 6-8.

DETAILED DESCRIPTION

One aspect of the present invention and will now be explained withreference to FIGS. 6A and 6B, which shows views of a two-dimensionalarray transducer 100 viewed in the azimuthal and elevational directions,respectively. As shown in FIG. 6A, the transducer 100 scans using adiverging center beam 102 and a separate pair of diverging side beams104, 106. Ultrasound echoes scanned by each of these beams 102, 104, 106are projected onto respective planes of projection 112, 114, 116. Pointsat corresponding depths in the planes of projection are then combined tocreate a single plane of projection that is used to create thevolumetric image. The plane of projection 112 can be used as the singleplane of projection by transferring points on the planes of projection114, 116 to the plane of projection 112 at the corresponding depth.

Significantly, the side beams 104, 106 scan to a ranges of distances 120from the transducer 100 that is greater than a ranges of distances 122that is scanned using the center beam 102. The difference between thescan distance of the center beam 102 and the scan distance of the sidebeams 104, 106 is selected so that both scan distances are atsubstantially the same depth beneath the transducer 100. As a result,the side beams 104, 106 and the center beam 102 scan to substantiallythe same depth. More specifically, as shown in FIG. 6A, when thetransducer 100 causes the center beam 102 to scan in the range ofdistances 122 from the transducer 100, all of the points in that rangeof distances 122 will be projected onto the plane of projection 112within a range of depths 126 that is only slightly smaller than theactual range of depths 128. At the same time, when the transducer 100causes the side beams 104, 106 to scan at the range of distances 120from the transducer 100, all of the points in that range 120 will beprojected onto the planes of projection 114, 116 as points fallingwithin the range although the actual locations of the points are in arange of depths 124. However, this range of depths 124 differs from therange of distances at which points are projected onto the planes 114,116 substantially less than in the conventional technique shown in FIGS.4A and 4B. As a result, when viewed in the elevational direction asshown in FIG. 6B, the depth of anatomical structures will be correctlyviewed with substantially less geometric distortion present using theconventional technique shown in FIGS. 4A and 4B. The advantage of usingside beams 104, 106 focused to a greater depth than the center beam 102will be apparent by comparing FIG. 6B with FIG. 4B.

Although the embodiment shown in FIGS. 6A and 6B uses only two sidebeams 104, 106, it will be understood that a larger number of side beamscould be used. Using a larger number of side beams further reduces thegeometric distortion that would otherwise be present, but it increasesthe processing that is required to display an image and may thereforepreclude real-time volumetric imaging. Alternatively, volumetric imagingcould be accomplished using two side-by-side diverging beams (notshown), but doing so would result in greater geometric distortion butless processing compared to the technique shown in FIGS. 6A and 6B. Ingeneral, scanning over a wider area or obtaining an image of greaterclarity makes it desirable to use a larger number of beams, particularlyif the processing power is available. Regardless of the number of beamsthat are used, the points on each plane of projection 112, 114, 116 arepreferably projected onto a single plane of projection with a weightcorresponding to the width of the respective beam. As a result, eachultrasound echo will be projected onto the plane of projection with thesame weight regardless of the beam 102-106 that obtained the echo.

The diverging beams 102, 104, 106 can be generated by thetwo-dimensional transducer 100 using a variety of techniques. The beams102-106 can be generated by operating array elements of the transducer100 in a phase-arrayed manner either in respective sub-arrays to formthe beams 102-106 at the same time or using all of the array elements ofthe transducer 100 to sequentially form each individual beam 102-106 atdifferent times. Also, the array elements can be arranged in sub-arrays,each of which is provided with a lens or other mechanical structure tocause a respective beam 102-106 to be generated from the sub-arrays.

One embodiment of another aspect of the present invention is illustratedin FIG. 7, which shows a two-dimensional array transducer 140 thattransmits and receives ultrasound and a plurality of sequentiallygenerated beams 142, 144, 146 for scanning within a respective range ofdepths. The angle of divergence of each beam 142-146 is inverselyrelated to the depth of its scanning range. Thus, the angle ofdivergence of the beam 142, which scans to a relatively shallow depth,is relatively wide, and the angle of divergence of the beam 146, whichscans to a relatively large depth, is relatively narrow. As a result,the width of each beam 142-146 at the furthest extent of its scan depthis substantially the same for all beams 142-146.

After ultrasound echoes have been obtained using the beams 142-146, avolumetric image is generated by using the echoes within the scan rangeof each beam 142-146. Thus, the image is generated from relativelyshallow echoes using the beam 142, moderately deep echoes using the beam144, and relatively deep echoes using the beam 146. The resulting imagecan encompass a width shown by the dotted lines 150, 152, which has asubstantially larger width than the image area encompassed by thecropping lines 86, 88 shown in FIG. 5.

A variety of techniques can be used to generate the beams 142-146 withdiffering divergence angles. However, the beams 142-146 are preferablygenerated by controlling the array elements of the transducer 140 usingphased-array techniques.

The technique shown in FIG. 7 can, of course, be used with a single beamscanning within the each range, or multiple beams can be used to scanwithin each range using the technique shown in FIGS. 6A and 6B.

One embodiment of still another aspect of the invention is shown inFIGS. 8A-8D. In this embodiment, the two-dimensional array elements of atransducer (not shown) are used to scan in relatively narrow beams inwhich all of the points at each range are projected onto a central planeof projection. For example, as shown in FIG. 8A, one volumetric scanningbeam 150 is used that is perpendicular to a second volumetric scanningbeam 152. The resulting projections 154, 156, respectively, show avessel in transverse cross-section 160 and longitudinal cross-section162, respectively.

As shown in FIG. 8B, two parallel scanning beams 170, 172 may be used togenerate respective transverse cross sectional projections 174, 176 of avolumetric region of a vessel 178 that are parallel to each other andspaced apart a predetermined distance.

Although the scaling of the projections 154, 156 and 174, 176 is uniformin the embodiments of FIGS. 8A and 8B, volumetric projections of ananatomical structure obtained using the same volumetric scanning beammay be shown with two different degrees of scaling, as shown in FIG. 8Cmore specifically, a single volumetric scanning beam 180 is used togenerate a first projection 182 showing a vessel 184 to actual scale anda second projection 186 showing the vessel 184 in expanded form. Thisembodiment can allow anatomical structures to be shown with greaterclarity.

Finally, FIG. 8D shows two volumetric scanning beams 190, 192intersecting each other at substantially the same angle that ananatomical structure 194 would be viewed by respective eyes. The beams190, 192 are used to generate a pair of image projections 196, 198 ofthe anatomical structure 194, which are viewed by respective eyes sothat the depth features of the anatomical structure can be visualized.

Although volumetric scanning beams having a variety of specificgeometric relationships have been illustrated in FIGS. 8A-8D, it will beunderstood that the use of a two-dimensional array transducer allows agreat deal of flexibility in the geometric relationships of scanningbeams that can be formed. Further, although FIGS. 8A-8D show only one ortwo volumetric scanning beams being used, it will be understood that agreater number of volumetric scanning beams can be used to create acorrespondingly greater number of projected images.

One potential limitation of the various embodiments of the inventivevolumetric scanning techniques may be the lack of resolution achievableat a specific depth. As mentioned above, all of the anatomicalstructures at the same depth are projected onto the same area of a planeof projection. Therefore, an anatomical structure occupying a relativelysmall width of the scanning beam may be masked or otherwise obscured byother anatomical structures at that same depth. To alleviate thispotential problem, three-dimensional scanning can be used to resolvespecific anatomical structures. The resulting image of such structurescan be overlaid onto the volumetric image. Significantly, the relativelylittle amount of processing power required to perform volumetricscanning in accordance with the various embodiments of the invention mayleave processing power available to perform three-dimensional scanningof limited areas without reducing the acquisition frame ratesignificantly. As a result, real-time imaging can still be achieved withthis limited amount of three-dimensional scanning to overlay volumetricscanning of a larger area.

One embodiment of an ultrasound imaging system 200 that can be used toperform volumetric imaging in accordance with the present invention isshown FIG. 9. The imaging system includes a probe 210 having atwo-dimensional array of transducer elements 212. The probe 210 iscoupled to through a cable 218 to a scanner 230.

The scanner 230 includes a transmitter 232, which generates highfrequency signals that are applied to the transducer elements 212 tocause the transducer elements 212 to transmit ultrasound into tissues orblood. Ultrasound echoes of the transmitted ultrasound are received bythe transducer elements 212, which generate corresponding analogsignals. These analog signals are applied to a preamplifier 234, whichamplifies the analog signals. The preamplifier 234 also includesinternal TGC (time gain control) circuitry to compensate for attenuationof the transmitted and received ultrasound at greater depths. Theamplified and depth compensated signals from the preamplifier 234 areapplied to an analog-to-digital (A/D) converter 238 where they aredigitized. The digitized echo signals are then formed into beams by abeamformer 244. The beamformer 244 is controlled by a controller 246,which is responsive to a user control. The controller 246 providescontrol signals to the transmitter 232 instructing the probe 210 as tothe timing, frequency, direction and focusing of transmit beams. Thecontroller 246 also controls the beamforming of the digitized echosignals received by the beamformer 244. The output of the beamformer 244is applied to an image processor 248, which performs digital filtering,B mode detection, and Doppler processing on the beamformed digitalsignals. The image processor 248 can also perform other signalprocessing such as harmonic separation, speckle reduction throughfrequency compounding, and other desired image processing.

Scanning to produce the volumetric images as explained with reference toFIGS. 6-8 is accomplished by the controller 246 controlling thebeamformer 244 so that it scans ultrasound echoes having theconfigurations of the beams shown in FIGS. 6-8. The controller 246 mayalso control the transmitter 232 so that it transmits ultrasound inbeams having the configuration shown in FIGS. 6-8. Since thetwo-dimensional array of transducer elements 214 has the ability tosteer transmitted and received beams in any direction and at anyinclination in front of the transducer 212, the beams can have anyorientation with respect to the transducer 212 and to each other.

The echo signals produced by the scanner 230 are coupled to the digitaldisplay subsystem 250, which processes the echo signals for display inthe desired image format. The digital display system 250 includes animage line processor 252, which is samples the echo signals and splicessegments of beams into complete line signals. The image line processoralso averages line signals for signal-to-noise improvement or flowpersistence. The image line signals from the image line processor 252are applied to a scan converter 254, where they are converted into thedesired image format. For example, the scan converter 254 may performRho-theta conversion as is known in the art. The image is then stored inan image memory 258 from which it can be displayed on a display 260. Theimage in the image memory 258 may also be overlaid with graphics to bedisplayed with the image. The graphics are generated by a graphicsgenerator 264, which is responsive to a user control. Individual imagesor image sequences can be stored in a cine memory 268 during capture ofimage loops.

For real-time volumetric imaging, the display subsystem 250 alsoincludes a three-dimensional image rendering processor 270, whichreceives image lines from the image line processor 252. Thethree-dimensional image rendering processor 270 renders of a real-timethree dimensional image, which is displayed on the display 260.

Although the present invention has been described with reference topreferred embodiments, persons skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A method of producing a volumetric ultrasound image, comprising:using a two-dimensional array transducer to scan a region of interest inan azimuthal direction using a plurality of beams that diverge in anelevational direction, the beams being positioned adjacent each other inthe elevational direction; projecting ultrasound reflections in eachbeam onto a plane of projection, the reflections being obtained in arange of distances from the transducer and being projected onto theplane of projection at corresponding distances from the transducer; andcreating the volumetric ultrasound image by combining the projections onthe plane of projection for each beam into a common plane of projection.2. The method of claim 1 wherein the act of using a two-dimensionalarray transducer to scan a region of interest in an azimuthal directionusing a plurality of beams that diverge in an elevational directioncomprises using each of a plurality of array elements aligned in theazimuthal direction to sequentially scan successive sub-regions regionof interest extending in the azimuthal direction.
 3. The method of claim1 wherein the act of using a two-dimensional array transducer to scan aregion of interest in an azimuthal direction comprises using each of aplurality of array elements aligned in the azimuthal direction tosequentially scan successive sub-regions extending in the azimuthaldirection in the region of interest.
 4. The method of claim 1 whereinthe act of using a two-dimensional array transducer to scan a region ofinterest in an azimuthal direction comprises using a plurality of arrayelements in a phased-array manner to steer each of the beams through arange of angles extending in the azimuthal direction.
 5. The method ofclaim 1 wherein the act of using a two-dimensional array transducer toscan a region of interest in an azimuthal direction using a plurality ofbeams that diverge in an elevational direction comprises using a centerbeam positioned between two side beams.
 6. The method of claim 1 whereineach of the beams scans a plurality of ranges of scanning depths usingrespective divergence angles that are ordered inversely to the ranges ofscanning depths so that when each of the beams scans its shallowestrange of scanning depths it has the largest divergence angle and wheneach of the beams scans its deepest range of scanning depths it has thesmallest divergence angle.
 7. The method of claim 1 wherein thevolumetric ultrasound image is created in real time.
 8. The method ofclaim 1, further comprising: using the two-dimensional array transducerto perform a three-dimensional scan of a portion of the region ofinterest; creating a three-dimensional ultrasound image from thethree-dimensional scan; and overlaying the three-dimensional ultrasoundimage on the volumetric ultrasound image.
 9. A method of producing avolumetric ultrasound image, comprising: using a two-dimensional arraytransducer to scan a region of interest in an azimuthal direction usinga plurality of beams that have a common center axis, the beams divergingin an elevational direction in respective divergence angles that aredifferent for each beam, the beams scanning respective ranges ofscanning depths that are ordered inversely to an order of divergenceangles of the beams so that a beam scanning the shallowest range ofscanning depths has the largest divergence angle and a beam scanning thedeepest range of scanning depths has the smallest divergence angle;projecting ultrasound reflections in each beam onto a common plane ofprojection, the reflections obtained for each beam being in therespective range of scanning depth; and creating the volumetricultrasound image from the ultrasound reflections projected onto thecommon plane of projection for all of the beams.
 10. The method of claim9 wherein all of the beams have substantially the same dimension in theelevational direction at the maximum depth in their respective ranges ofscanning depths.
 11. The method of claim 9 wherein the volumetricultrasound image is created in real time.
 12. The method of claim 9,further comprising: using the two-dimensional array transducer toperform a three-dimensional scan of a portion of the region of interest;creating a three-dimensional ultrasound image from the three-dimensionalscan; and overlaying the three-dimensional ultrasound image on thevolumetric ultrasound image.
 13. A method of producing a volumetricultrasound image, comprising: using a two-dimensional array transducerto scan a region of interest in an azimuthal direction using a beam thatdiverges in an elevational direction, the beam scanning a plurality ofranges of scanning depths using respective divergence angles that areordered inversely to the ranges of scanning depths so that when the beamscans the shallowest range of scanning depths it has the largestdivergence angle and when the beam scans the deepest range of scanningdepths it has the smallest divergence angle; projecting ultrasoundreflections at each range of scanning depths onto a plane of projection;and creating the volumetric ultrasound image from the ultrasoundreflections projected onto the plane of projection.
 14. The method ofclaim 13 wherein the beam has substantially the same dimension in theelevational direction at the maximum depth in each of the ranges ofscanning depths.
 15. The method of claim 13 wherein the volumetricultrasound image is created in real time.
 16. The method of claim 13,further comprising: using the two-dimensional array transducer toperform a three-dimensional scan of a portion of the region of interest;creating a three-dimensional ultrasound image from the three-dimensionalscan; and overlaying the three-dimensional ultrasound image on thevolumetric ultrasound image.
 17. A method of producing a volumetricultrasound image, comprising: using a two-dimensional array transducerto scan a region of interest using a pair of beams, a first of the beamsdiverging in a first direction and being used to scan the region ofinterest in a second direction that is perpendicular to the firstdirection, a second of the beams diverging in a third direction andbeing used to scan the region of interest in a fourth direction that isperpendicular to the third direction; projecting ultrasound reflectionsin the first beam onto a plane of projection that is perpendicular tothe first direction; projecting ultrasound reflections in the secondbeam onto a plane of projection that is perpendicular to the thirddirection; and creating the volumetric ultrasound image from the firstand second planes of projection.
 18. The method of claim 17 wherein thesecond direction is parallel to the third direction so that the firstand second planes of projection are parallel to each other.
 19. Themethod of claim 17 wherein the second direction is perpendicular to thethird direction so that the first and second planes of projectionintersect each other at a right angle.
 20. The method of claim 17wherein the volumetric ultrasound image is created in real time.
 21. Anultrasound diagnostic imaging system comprising: a two-dimensional arraytransducer; a beamformer coupled to the two-dimensional array transducerto beamform received ultrasound echo signals; a controller coupled tothe two-dimensional array transducer, the controller controlling thetwo-dimensional array transducer to scan a region of interest in anazimuthal direction using a plurality of beams that diverge in anelevational direction, the beams being positioned adjacent each other inthe elevational direction; a processor processing the beamformedultrasound echo signals and projecting ultrasound echoes scanned by eachbeam onto a respective plane of projection; and a display subsystemcoupled to the processor, the display subsystem creating a volumetricultrasound image by combining the projections on the plane of projectionfor each beam into a common plane of projection.
 22. The system of claim21 wherein the controller controls the two-dimensional array transducerto scan a region of interest in an azimuthal direction by using each ofa plurality of array elements in the two dimensional array transducerthat are aligned in the azimuthal direction to sequentially scansuccessive sub-regions region of interest extending in the azimuthaldirection.
 23. The system of claim 21 wherein the controller controlsthe two dimensional array transducer to scan a region of interest in anazimuthal direction by using a plurality of array elements in thetwo-dimensional array transducer in a phased-array manner to steer eachof the beams through a range of angles extending in the azimuthaldirection.
 24. The system of claim 21 wherein the controller controlsthe two dimensional array transducer so that each of the beams scans aplurality of ranges of scanning depths using respective divergenceangles that are ordered inversely to the ranges of scanning depths sothat when each of the beams scans its shallowest range of scanningdepths it has the largest divergence angle and when each of the beamsscans its deepest range of scanning depths it has the smallestdivergence angle.
 25. The system of claim 21 wherein the volumetricultrasound image is created in real time.
 26. An ultrasound diagnosticimaging system comprising: a two-dimensional array transducer; abeamformer coupled to the two-dimensional array transducer to beamformreceived ultrasound echo signals; a controller coupled to thetwo-dimensional array transducer, the controller controlling thetwo-dimensional array transducer to scan a region of interest in anazimuthal direction using a plurality of beams that have a common centeraxis, the beams diverging in an elevational direction in respectivedivergence angles that are different for each beam, the controllercausing the beams to scan respective ranges of scanning depths that areordered inversely to an order of divergence angles of the beams so thata beam scanning the shallowest range of scanning depths has the largestdivergence angle and a beam scanning the deepest range of scanningdepths has the smallest divergence angle; a processor processing thebeamformed ultrasound echo signals and projecting ultrasound echoesscanned by each beam onto a common plane of projection, the ultrasoundechoes scanned by each beam being in the respective range of scanningdepth; and a display subsystem coupled to the processor, the displaysubsystem creating a volumetric ultrasound image from the ultrasoundechoes projected onto the plane of projection for all of the beams. 27.The system of claim 26 wherein the controller controls thetwo-dimensional array transducer so that all of the beams havesubstantially the same dimension in the elevational direction at themaximum depth in their respective ranges of scanning depths.
 28. Thesystem of claim 26 wherein the volumetric ultrasound image is created inreal time.
 29. An ultrasound diagnostic imaging system comprising: atwo-dimensional array transducer; a beamformer coupled to thetwo-dimensional array transducer to beamform received ultrasound echosignals; a controller coupled to the two-dimensional array transducer,the controller controlling the two-dimensional array transducer to scana region of interest using a pair of beams, a first of the beamsdiverging in a first direction and being used to scan the region ofinterest in a second direction that is perpendicular to the firstdirection, a second of the beams diverging in a third direction andbeing used to scan the region of interest in a fourth direction that isperpendicular to the third direction; a processor processing thebeamformed ultrasound echo signals and projecting ultrasound echoesscanned by the first beam onto a plane of projection that isperpendicular to the first direction and projecting ultrasound echoesscanned by the second beam onto a plane of projection that isperpendicular to the third direction; a display subsystem coupled to theprocessor, the display subsystem creating a volumetric ultrasound imagefrom the first and second planes of projection.
 30. The system of claim29 wherein the second direction is parallel to the third direction sothat the first and second planes of projection are parallel to eachother.
 31. The system of claim 29 wherein the second direction isperpendicular to the third direction so that the first and second planesof projection intersect each other at a right angle.